Various types of batteries have been developed heretofore, and in every battery, a packaging material is an essential member for sealing battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore for battery packaging, but in recent years, batteries have been required to be diversified in shape, and desired to be thinner and lighter as performance of electric cars, hybrid electric cars, personal computers, cameras, mobile phones and so on has been enhanced. However, metallic battery packaging materials that have often been heretofore used have the disadvantage that it is difficult to keep up with diversification in shape, and there is a limit on weight reduction.
Thus, in recent years, a film-shaped laminate including a base material layer, an adhesive layer, a barrier layer and a sealant layer laminated in this order has been proposed as a battery packaging material which is easily processed into diverse shapes and which can be thinned and lightened (see, for example, Patent Document 1). Such a film-shaped battery packaging material is formed in such a manner that a battery element can be sealed by heat-welding the peripheral edge of the sealant layers by heat sealing with the sealant layers facing each other.
In conventional film-shaped battery packaging materials, the barrier layer serves not only to improve the strength of the packaging material but also to prevent ingress of water vapor, oxygen, light and so on into a battery, and metal foils are often used as the barrier layer. These metal foils have low adhesiveness with other layers, so that layer delamination easily occurs. Usually, an electrolytic solution does not contain water, but if water is mixed in the electrolytic solution with some cause, hydrogen fluoride may be generated to corrode or dissolve a metal foil. Thus, conventionally, one or both of the surfaces of a metal foil to be laminated in a film-shaped battery packaging material is subjected to a chromate treatment to impart electrolytic solution resistance to the metal foil, so that bonding of the metal foil to other layers is stabilized, corrosion and dissolution of the metal foil are prevented, and so on. However, performance requirements for film-shaped battery packaging materials are diversified, and it is desired to develop a technique for improving electrolytic solution resistance using a method other than the chromate treatment of a metal foil.
If very small contaminants such as debris of an electrode active material and an electrode tab stick to the surface of the sealant layer in the production process of batteries, the very small contaminants may be brought into contact with the barrier layer by heat and pressure during heat sealing, leading to occurrence of a short-circuit. Accordingly, in the film-shaped battery packaging material, it is required to improve safety by maintaining a high insulation quality even if such very small contaminants enter the battery packaging material. Thus, it is earnestly desired to develop a technique for imparting both excellent electrolytic solution resistance and an excellent insulation quality to the film-shaped battery packaging material.